Mechanism of quercetin oxygenation a possible model for haem degradation

The case for an oxidopyrylium intermediate in the mechanism of quercetin dioxygenases

The quercetin dioxygenases (QDOs) are unusual metalloenzymes in that they display ring-opening dioxygenase activity with several different first-row transition metal ions which do not undergo redox changes during turnover. The QDOs are also unique in that the substrate binds as an η1-flavonolate rather than the η2 -bidentate mode seen in all reported model complexes. The flavonol substrates were early examples of excited state intramolecular proton transfer (ESIPT) phenomena, in which photoexcitation causes an H-atom exchange between the adjacent hydroxyl and ketone, generating an oxidopyrylium emissive state. These oxidopyryliums undergo ring-opening dioxygenations analogous to the enzymatic reactions. Our hypothesis is that lability of the divalent metal ion may allow access to a reactive oxidopyrylium intermediate via coordination switching from the oxy to ketone position, which allows reaction with O2. In this report, we use a straight-forward methylation strategy to generate a panel of flavonol and thioflavonol derivatives modeling several η1- and η2-coordination modes. Methylation of 3-hydroxythioflavone generates an air stable η1 hydroxopyrylium salt, which undergoes rapid ring-opening dioxygenation by deprotonation or photoexcitation. By comparison, the η1-methoxyflavonol does not react with O2 under any condition. We find that any of the studied flavonol derivatives, η1 or η2, which demonstrates ESIPT-like oxidopyrylium emissions undergo QDO-like ring-opening reactions with dioxygen. The implications of these results concerning the mechanism of QDOs and related dioxygenases is discussed.

Acireductone Dioxygenase- (ARD-) Type Reactivity of a Nickel(II) Complex Having Monoanionic Coordination of a Model Substrate: Product Identification and Comparisons to Unreactive Analogues

A mononuclear Ni(II) complex ([(6-Ph2TPA)Ni(PhC(O)C(OH)C(O)Ph)]ClO4 (1)), supported by the 6-Ph2TPA chelate ligand (6-Ph2TPA = N,N-bis((6-phenyl-2-pyridyl)methyl)-N-((2-pyridyl)methyl)amine) and containing a cis-beta-keto-enolate ligand having a C2 hydroxyl substituent, undergoes reaction with O2 to produce a Ni(II) monobenzoate complex ([(6-Ph2TPA)Ni(O2CPh)]ClO4 (3)), CO, benzil (PhC(O)C(O)Ph), benzoic acid, and other minor unidentified phenyl-containing products. Complex 3 has been identified through independent synthesis and was characterized by X-ray crystallography, 1H NMR, FAB-MS, FTIR, and elemental analysis. A series of cis-beta-keto-enolate Ni(II) complexes supported by the 6-Ph2TPA ligand ([(6-Ph2TPA)Ni(PhC(O)CHC(O)Ph)]ClO4 (4), [(6-Ph2TPA)Ni(CH3C(O)CHC(O)CH3)]ClO4 (5), and [(6-Ph2TPA)Ni(PhC(O)CHC(O)C(O)Ph) (6)) have been prepared and characterized. While these complexes exhibit structural and/or spectroscopic similarity to 1, all are unreactive with O2. The results of this study are discussed in terms of relevance to Ni(II)-containing acireductone dioxygenase enzymes, as well as in the context of recently reported cofactor-free, quercetin, and beta-diketone dioxygenases.

Facile synthesis of flavonoid 7-O-glycosides

Highly regioselective removal of the 7-O-acyl groups of the peracylated flavones, isoflavones, and flavonols (PhSH, imidazole, NMP) followed by effective glycosylation with glycosyl trifluoroacetimidates (BF(3).Et(2)O) and cautious deprotection of the acyl groups under basic conditions afforded the desired 7-O-flavonoid glycosides in satisfactory yields.

Kinetics and mechanism of the base-catalyzed oxygenation of flavonol in DMSO-H(2)O solution

The kinetics of the base-catalyzed oxygenation of flavonol have been investigated in 50% DMSO-H(2)O solution in the pH range 6.4-10.8 and an ionic strength of 0.1 mol L(-1) using spectrophotometric techniques at temperatures between 70 and 90 degrees C. The rate law -d[flaH]/dt = k(obs) [OH(-)][flaH][O(2)] (k(obs) = kK(1)/[H(2)O]) describes the kinetic data. The rate constant, activation enthalpy, and entropy at 353.16 K are as follows: k/mol(-1) L s(-1) = (4.53 +/- 0.07) x 10(-2), DeltaH/kJ mol(-1)= 59 +/- 4, DeltaS/J mol(-1) K(-1) = -110 +/- 11. The reaction showed specific base catalysis. It fits a Hammett linear free energy relationship for 4'-substituted flavonols and electron-releasing substituents enhanced the reaction rate. The linear correlation between the oxidation potential of the flavonols and the rate constants supports that a higher electron density on the flavonolate ion makes them more nucleophilic and the electrophilic attack of O(2) easier.

Kinetics and mechanism of the oxygenation of potassium flavonolate. Evidence for an electron transfer mechanism

The oxygenation of the potassium salt of flavonol (flaH) in absolute DMF leads to potassium O-benzoylsalicylate and carbon monoxide in 95% yield at 40 degrees C. Kinetic measurements resulted in the rate law -d[flaK]/dt = k(2)[flaK][O(2)]. The rate constant, activation enthalpy, and entropy at 313.16 K are as follows: k(2)/M(-)(1) s(-1) = (3.28 +/- 0.10) x 10(-1), DeltaH()/kJ mol(-1) = 29 +/- 2, DeltaS/J mol(-1) K(-1) = -161 +/- 6. The reaction fits a Hammett linear free energy relationship for 4'-substituted flavonols, and electron-releasing groups make the oxygenation reaction faster. The anodic oxidation wave potentials E(a) of the 4'-substituted flavonolates correlate well with reaction rates. At more negative E(a) values faster reaction rates were observed. EPR spectrum of the reaction mixture (g = 2.0038, dH = 1.8 G, a(H) = 0.9 G) showed the presence of flavonoxyl radical as a result of a SET from the flavonolate to dioxygen.

Fate of the flavonoid quercetin in human cell lines: chemical instability and metabolism

Although cell cultures are increasingly being used as models for studying the biological actions of flavonoids, no information on the fate, such as uptake and metabolism, exists for these natural products in these models. This study examined the elimination of quercetin, one of the most abundant flavonoids, from the cultured human hepatocarcinoma cell line Hep G2 using [14C]-labelled compound with HPLC and LC/MS for structure characterization. These cells showed a 9.6-fold accumulation of quercetin and the formation of an O-methylated metabolite, isorhamnetin. However, a rapid elimination of quercetin, with no unchanged compound present beyond 8 h, was mainly due to oxidative degradation. The initial intermediate reaction appears to involve peroxidation, leading to a dioxetan, as evidenced by a 32-amu increase in the molecular ion by LC/MS. Subsequently, opening of the C-ring leads to the formation of carboxylic acids, the major one identified in this study as protocatechuic acid. A separate reaction results in a polymeric quercetin product which is highly retained on a reversed-phase C18 HPLC column. It is postulated that these degradative and metabolic changes contribute to the multiple biological actions reported for quercetin, using cell culture models. Interestingly, part of the degradative pathway could be inhibited by including nontoxic concentrations of EDTA in the cell culture medium.

Urinary and faecal mutagenicity in Sprague-Dawley rats dosed with the food mutagens quercetin and rutin

The natural flavonoid quercetin was administered to Sprague-Dawley rats by ip injection or gastric intubation of a single dose of 500, 1000 or 2000 mg/kg body weight. Mutagenicity assays with Salmonella typhimurium strain TA98 showed moderate mutagenic activity in the urines and faecal extracts but not in plasma samples from the treated animals. The mutagenic activity detected in the urines accounted for about 0.5% of the administered dose, irrespective of the route of administration and the dose level. Higher mutagenicity was demonstrated in faecal extracts. Rutin (quercetin-3-O-rutinoside) was administered by gavage and ip injection at 2000 mg/kg. Although the chemical was inactive as a mutagen in vitro, significant mutagenicity was detected in the urines and faecal extracts of the treated rats. Such activity was similar to that detected after administration of free quercetin in a dose some four times lower (by weight).

Factors which facilitate inactivation of quercetin mutagenicity

Oxygen, oxidizing enzymes such as polyphenol oxidase (tyrosinase) and alkaline pH, irreversibly inactivate the mutagenicity of quercetin in the Ames test. The loss of mutagenic activity correlates with decreases in the ultraviolet absorption maximum of quercetin near 370 nm. The extent of inactivation increases with time but apparently not significantly with temperature of exposure, and decreases with quercetin concentration. Metal salts such as ferrous and copper sulfates also facilitate inactivation, but these effects may be reversible. Understanding the factors which minimize the mutagenic potential of food ingredients should lead to safer foods.